Processing of fibrous magnesium silicate coated silicon steel

ABSTRACT

DESCRIBED HEREIN IS AN IMPROVEMENT IN THE PROCESSING OF SILICON STEEL WHERE THE STEEL IS COATED PRIOR TO HEAT TREATMENT WHICH INVOLVES THE APPLICATION OF A COATING OF FIBROUS MAGNESIUM SILICATE INSTEAD OF MAGNESIA WHICH HAS BEEN USED PREVIOUSLY AND WHICH RESULTED IN THE IN SITU FORMATION OF CRYSTALLINE MAGNESIUM SILICATE.

United States Patent 3,562,029 PROCESSING OF FIBROUS MAGNESIUM SILICATECOATED SILICON STEEL James J. Goglio, Leechburg, and Clarence L. Miller,Jr., Pittsburgh, Pa., assignors to Allegheny Ludlum Steel Corporation,Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed Apr.18, 1968, Ser. No. 722,225

Int. Cl. C04b 35/58; H01f 1/18 US. Cl. 148-113 3 Claims ABSTRACT OF THEDISCLOSURE Described herein is an improvement in the processing ofsilicon steel where the steel is coated prior to heat treatment whichinvolves the application of a coating of fibrous magnesium silicateinstead of magnesia which has been used previously and which resulted inthe in situ formation of crystalline magnesium silicate.

This invention relates to a method of treating silicon steel. Moreparticularly, the invention concerns a method for minimizing orrecluding the formation of an abrasive coating on the surface of saidsteel when exposed to elevated temperatures.

Silicon steel suitable for electrical applications is typically heattreated in coil form during the processing thereof to develop graingrowth and magnetic properties. Prior to heat treating a coil of siliconsteel, the surface is coated with magnesia to prevent welding and toallow proper magnetic properties to develop. During annealing themagnesia reacts with silica present on the surface of the steel to forma complex coating containing crystalline magnesium silicate. Thiscoating, in the form normally occurrng, is moderately insulating, glassyin appearance, and typically bluish gray in coloring. It is also highlyabrasive and can detrimentally affect the life of dies used in thefabrication of the silicon steel having such a coating on its surface. Acommon method of fabricating such steel is by stamping, and theabrasiveness of the glassy magnesium silicate coating causes excessivedie wear in such an operation.

The present invention has an an object preventing the formation of theabrasive crystalline magnesium silicate coating on the surface ofsilicon steel during high temperature exposure as in heat treating. Theformation of the abrasive coating is prevented by applying to thesurface of the silicon steel, prior to exposure to elevatedtemperatures, a layer of fibrous magnesium silicate. This layer preventsthe formation of crystalline magnesium silicate, and also preventsWelding of adjacent laps of the silicon steel coil together duringannealing. The fibrous magnesium silicate does not interfere with themetallurgical treatment to develop the desired magnetic properties.

The effectiveness of the fibrous magnesium silicate in preventing orminimizing the formation of crystilline magnesium silicate can bedetermined by measuring the electrical resistance of the silicon steelsurface after heat treatment. Since, as mentioned above, the crystallinemagnesium silicate, which may form by reaction of the magnesia coatingand silica present on the surface, is moderately insulating, the extentof its formation is reflected by measuring the surface electricalresistance. A useful test for this purpose is the so-called Franklintest of interlamination resistance which is described in ASTMSpecification A344-60T (Method B). In this test, electrical conductivityof the surface is measured in amps, and the higher the amp value thegreater the conductivity and consequently the lower the resistancecaused by the presence of crystalline magnesium silicate on the surface.This test is used as an indication of the extent of the base reactionbetween silica and magnesia.

The following examples will illustrate the usefulness of fibrousmagnesium silicate in reducing the formation of the crystallinemagnesium silicate as measured by the aforementioned Franklin test ofinterlamination resistance.

A series of steel samples of uniform size and composition were providedwith coatings of different concentration of fibrous magnesium silicate.The coated samples were then annealed at 2150 F. to simulate normalannealing of silicon steel for the purpose of developing grain growth.These samples were also tested for electrical properties to determinewhether the fibrous magnesium silicate coating interfered with thedevelopment of the required properties. The results of the Franklin teston these samples are described in Table I.

As can be seen from the data in Table I, the resistance decreasesmarkedly when sufiicient fibrous magnesium silicate is present. Aconvenient guide for indicating that the formation of crystallinemagnesium silicate has not exceeded acceptable limits is a Franklin testvalue of greater than 0.9 amp. In accordance with the invention,sufficient magnesium silicate must be present in the coating applied tothe silicon steel surface to provide a Franklin test result greater than0.9 amp.

It has been mentioned above that the use of the fibrous magnesiumsilicate coating does not detrimentally affect the development of therequired electrical properties. This is demonstrated by the data inTable II which reports the results of permeability and core loss testson samples of the type described above containing fibrous magnesiumsilicate, and a control sample Without the fibrous silicate coating. Themagnetic pro erties reported in Table II were made on Epstein packs ofsamples (3 cm. x 30.5 cm.).

1 Watts per pound. 2 B =gausses, KB =1,000 gausscs. 3 Oersteds.

The absence of crystalline magnesium silicate following heat treatment,i.e. annealing, is verified by visual observation as well as theFranklin test indication as discussed previously. Crystalline magnesiumsilicate which forms by reaction of magnesia with silica present on thesurface of the silicon steel is typically bluish gray in color andglassy in appearance. The surface of the steel after treating withfibrous magnesium silicate and after high temperature exposure has ametallic appearance and remains clear and colorless.

Advantageously, the coating is applied substantially continuously on themetal surface and in coating weights of from about 0.01 to 0.04 oz./ft.Although any suitable method of application may be used, one presentlypreferred technique is to apply the coating by dipping and metering withapproved rubber rolls.

'It is apparent from the above that various changes and modificationsmay be made without departing from the invention. Accordingly, the scopeof the invention should be limited only by the appended claims.

We claim:

1. In the processing of silicon steel, wherein said steel is exposed toelevated temperature prior to subsequent fabrication, the improvementcomprising applying to a surface of said steel prior to exposure thereofto elevated temperature a coating cotaining fibrous magnesium silicatein an amount sufficient to produce, after exposure to said elevatedtemperature, Franklin test result of greater than about 0.9 amp and ametallic appearance on the surface.

2. An improvement according to claim 1 wherein said coating contains atleast 50% by Weight fibrous magnesium silicate.

3. An improvement according to claim 1 wherein said fibrous magnesiumsilicate is applied at a coating weight of from about 0.01 to 0.04oz./ft.

References Cited L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE,Assistant Examiner US. Cl. X.R.

